A material, derived from human erythrocytes and believed to be identical with the receptor site for the myxoviruses, has been obtained in homogeneous form. The method of preparation involves pH adjustment of the stroma, hot phenolic extraction, chloroform-methanol treatment, and ultracentrifugation. The material so obtained possesses a high inhibitory titer to viral hemagglutination (45,000 to 60,000 inhibitory units/mg. against 4 hemagglutination units of Lee strain of influenza virus) and appears to be a glycoprotein containing 22 to 24 per cent sialic acid, 12 per cent hexose, 12 per cent hexosamine, 1 per cent fucose in addition to at least eleven amino acids. The sialic acid probably occupies a terminal position in an oligosaccharide chain extending from the protein peptide chain. The molecular weight is near 30,000—an unusually low value for substances possessing this biological activity. M and N blood group activity also seems to be associated with this protein.

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